Ensure each cache config test uses separate state.

The tests modify global state (`_CACHES`), by patching this in the
test cases, each test gets a separate dictionary, which is discarded
at the end of the test.

tests/config/test_cache.py

@@ -12,12 +12,16 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from unittest.mock import patch
+
 from synapse.config.cache import CacheConfig, add_resizable_cache
 from synapse.util.caches.lrucache import LruCache
 
 from tests.unittest import TestCase
 
 
+# Patch the global _CACHES so that each test runs against its own state.
+@patch("synapse.config.cache._CACHES", new_callable=dict)
 class CacheConfigTests(TestCase):
     def setUp(self):
         # Reset caches before each test
@@ -26,7 +30,7 @@ def setUp(self):
     def tearDown(self):
         self.config.reset()
 
-    def test_individual_caches_from_environ(self):
+    def test_individual_caches_from_environ(self, _caches):
         """
         Individual cache factors will be loaded from the environment.
         """
@@ -39,7 +43,7 @@ def test_individual_caches_from_environ(self):
 
         self.assertEqual(dict(self.config.cache_factors), {"something_or_other": 2.0})
 
-    def test_config_overrides_environ(self):
+    def test_config_overrides_environ(self, _caches):
         """
         Individual cache factors defined in the environment will take precedence
         over those in the config.
@@ -56,7 +60,7 @@ def test_config_overrides_environ(self):
             {"foo": 1.0, "bar": 3.0, "something_or_other": 2.0},
         )
 
-    def test_individual_instantiated_before_config_load(self):
+    def test_individual_instantiated_before_config_load(self, _caches):
         """
         If a cache is instantiated before the config is read, it will be given
         the default cache size in the interim, and then resized once the config
@@ -72,7 +76,7 @@ def test_individual_instantiated_before_config_load(self):
 
         self.assertEqual(cache.max_size, 300)
 
-    def test_individual_instantiated_after_config_load(self):
+    def test_individual_instantiated_after_config_load(self, _caches):
         """
         If a cache is instantiated after the config is read, it will be
         immediately resized to the correct size given the per_cache_factor if
@@ -85,7 +89,7 @@ def test_individual_instantiated_after_config_load(self):
         add_resizable_cache("foo", cache_resize_callback=cache.set_cache_factor)
         self.assertEqual(cache.max_size, 200)
 
-    def test_global_instantiated_before_config_load(self):
+    def test_global_instantiated_before_config_load(self, _caches):
         """
         If a cache is instantiated before the config is read, it will be given
         the default cache size in the interim, and then resized to the new
@@ -100,7 +104,7 @@ def test_global_instantiated_before_config_load(self):
 
         self.assertEqual(cache.max_size, 400)
 
-    def test_global_instantiated_after_config_load(self):
+    def test_global_instantiated_after_config_load(self, _caches):
         """
         If a cache is instantiated after the config is read, it will be
         immediately resized to the correct size given the global factor if there
@@ -113,7 +117,7 @@ def test_global_instantiated_after_config_load(self):
         add_resizable_cache("foo", cache_resize_callback=cache.set_cache_factor)
         self.assertEqual(cache.max_size, 150)
 
-    def test_cache_with_asterisk_in_name(self):
+    def test_cache_with_asterisk_in_name(self, _caches):
         """Some caches have asterisks in their name, test that they are set correctly."""
 
         config = {
@@ -139,7 +143,7 @@ def test_cache_with_asterisk_in_name(self):
         add_resizable_cache("*cache_c*", cache_resize_callback=cache_c.set_cache_factor)
         self.assertEqual(cache_c.max_size, 200)
 
-    def test_apply_cache_factor_from_config(self):
+    def test_apply_cache_factor_from_config(self, _caches):
         """Caches can disable applying cache factor updates, mainly used by
         event cache size.
         """